This invention relates to an optical sensor for measuring oxygen in a medium, provided with a substrate in which an organometallic complex is embedded.
The organometallic complex is an oxygen-sensitive fluorescent dye, with the amount of fluorescence and the fluorescence life being dependent on the oxygen content in the medium. Such an organometal typically consists of Tris-Ru2+-4,7-biphenyl-1,10-phenanthroline; this Ru(ruthenium) complex is particularly oxygen-sensitive, but other organometals can also be used, such as an Os complex or a Pt complex.
The organometallic complex is normally adsorbed to a silica gel. The silica gel can adsorb a high concentration of the dye without the fluorescent properties of the material being thereby affected. The silica gel with the adsorbed dye is embedded in a substrate of polymeric material, for instance a mixture of PDMS (polydimethyl-siloxane) and PTMSP (polytrimethylsilylpropyl), which polymers are gas permeable to a high degree, so that the response of the sensor to oxygen content changes can be prompt. By being embedded in the substrate, the organometallic complex is rendered insensitive to disturbing influences, such as, for instance, the action of moisture or leaching of the fluorescent component.
Through fluorescence measurements, the level of the oxygen content in the medium can be determined. Such a measurement is relatively simple to carry out, but has as a disadvantage that the measuring results, owing to the occurrence of, for instance, photobleaching or ageing of the sensor due to high temperatures, are no longer reproducible with the passage of time.
This phenomenon occurs in particular if measurements are performed where the medium consists of consumable oil, such as fish oil, sunflower oil, etc. In practice, there is an interest in determining the oxygen content in such media for the purpose of assessing the storage life thereof. Through chemical action of the medium, however, measurements with the sensor applied heretofore have been found to become unreliable with the passage of time.
This phenomenon also occurs as a result of a high temperature loading of the sensor, for instance when using the sensor as a feedback for the gas-air ratio in combustion apparatus. Further, this phenomenon occurs if the sensor is exposed for a relatively long time, for instance in the case of oxygen content measurements in groundwater.
It is attempted to obviate these problems by stabilizing the sensor. This has shown that an inherently chemically stable and gas permeable substrate material is not straightforwardly satisfactory: for that, the chemical interaction with the embedded dye, which must retain its oxygen-sensitive properties, and the substrate is too complex. To date, therefore, there is not any substrate known which, in combination with the oxygen-sensitive dye, continues to retain its favorable properties.